Nitrogen Mineralization and Availability at Critical Stages of Rice (Oryza sativa) Crop, and Its Relation to Soil Biological Activity and Crop Productivity Under Major Nutrient Management Systems

Adequate nitrogen availability to plants for growth is one of the most important reasons for fertilizer application. Though organic alternatives are recommended, there is uncertainty of their nutrient release characteristics, especially during critical growth stages of a crop. In a 10-year-long experiment on nutrient management for rice-wheat cropping, ion exchange resin (IER) membrane strips were used as plant root simulators to determine daily NH4+-N and NO3--N availability in soil solution during the rice growing season. The management included inorganic fertilizers at 100% recommended rate (F), compared to reduced rate (55%) of inorganic fertilizers supplemented with organic inputs via green manuring withSesbania(GM), biomass incorporation of an opportunity legume crop-green gram (Vigna radiata) (LE), 1/3rd wheat stubble retention and soil incorporation (WS), 1/3rd rice stubble retention and soil incorporation (RS), and farmyard manure application (FYM). The total amount of available N (NH4++ NO3-) recorded for the full season was in the order GM (221 mu g cm(-2)) > F (184 mu g cm(-2)) > RS (181 mu g cm(-2)) > FYM (176 mu g cm(-2)) > WS (176 mu g cm(-2)) > LE (175 mu g cm(-2)). Both grain and straw yield related directly and significantly to the N mineralization in soil at 30-60 days after transplanting (DAT), indicating that fertilizer N application before 30 DAT and after 60 DAT could mostly be a loss in transplanted rice crop. Green manured (GM) soils maintained steadily high N mineralization rates throughout the rice growing period. The best alternative to cut down inorganic fertilizer use in rice cropping would be the biomass incorporation from leguminous green manuring crops. Integration of organics afforded almost 50% reduction in recommended inorganic fertilizer use while maintaining better N mineralization status at the critical growth stages of rice.